Merge branch 'empty-paths' into 'master'

arkindex/dataimport/models.py

arkindex/dataimport/tests/test_process_elements.py

arkindex/documents/api/elements.py

arkindex/documents/export/element_path.sql

arkindex/documents/fixtures/data.json

arkindex/documents/management/commands/build_fixtures.py

arkindex/documents/management/commands/load_export.py

arkindex/documents/migrations/0048_empty_paths.py

arkindex/documents/migrations/0049_empty_path_constraints.py

arkindex/documents/models.py

arkindex/documents/serializers/elements.py

arkindex/documents/tests/commands/test_load_export.py

arkindex/documents/tests/tasks/test_export.py

arkindex/documents/tests/tasks/test_move_element.py

arkindex/documents/tests/test_corpus_elements.py

arkindex/documents/tests/test_create_elements.py

arkindex/documents/tests/test_edit_elementpath.py

arkindex/documents/tests/test_element_paths_api.py

arkindex/project/tests/test_arrayfield.py

arkindex/project/tools.py